1. Field of the Invention
The present invention relates to a humidity sensor, and more particularly, to a capacitive humidity sensor.
2. Description of the Related Art
A structure in which electrodes for capacitance measurement are formed on a semiconductor substrate and covered with a humidity sensitive film has been known as a related-art capacitive humidity sensor. The humidity sensitive film is characterized by having a dielectric constant that varies depending on the level of humidity, and detects a change in humidity from a change in capacitance between the electrodes. FIG. 9 is a diagram for illustrating an example of the sectional structure of the humidity sensitive film. An insulating film 2 is formed on a substrate 1, and a plurality of capacitance measurement electrodes 3a and a plurality of capacitance measurement electrodes 3b are formed on the insulating film 2. A thin protective film 4 is configured to cover surfaces of the electrodes in order to secure the reliability of the electrodes, and a thick humidity sensitive film 5 is configured to cover the protective film 4.
Known arrangement patterns of the capacitance measurement electrodes 3a and 3b according to the related art are illustrated in FIG. 10A to FIG. 10C. In FIG. 10A, two electrodes, namely, one electrode 3a and one electrode 3b, are arranged in a comb-teeth pattern (see Japanese Patent Translation Publication No. 2003-516539, for example). In FIG. 10B, a plurality of electrodes 3a each aligned to form an island shape are surrounded by the other electrode 3b (see Japanese Patent Application Laid-open No. 2007-192622, for example). The electrodes 3a each forming an island shape are electrically connected to one another through lower-layer wiring 6a. In FIG. 10C, a plurality of columnar electrodes 3a and a plurality of columnar electrodes 3b are aligned to form an equilateral triangle grid (see Japanese Patent Application Laid-open No. 2014-167445, for example). The columnar electrodes 3a are electrically connected to one another through the lower-layer wiring 6a, and the columnar electrodes 3b are electrically connected to one another through lower-layer wiring 6b. 
The comb-teeth wiring structure of FIG. 10A has an advantage in that the structure is high in capacitance per unit area and can be formed with a single layer of wiring. However, the layout slanted toward a particular direction may cause the humidity sensitive film to peel easily under stress applied in the particular direction. In addition, the structure has a weakness with regards to the ease of embedding in that a void is likely to, form when the humidity sensitive film is embedded in the case where the capacitance measurement electrodes 3a and 3b are desirably formed thick and a narrow space is desirably set between the electrodes in order to secure a desired capacitance in a limited area.
The structure of FIG. 10B is an electrode arrangement that has fourfold symmetry, and therefore has less chance for the humidity sensitive film to peel with respect to a particular direction. However, the likelihood of a void being formed in this structure is about the same as in the comb-teeth wiring structure of FIG. 10A. The structure of FIG. 10B is lower in capacitance per unit area than the comb-teeth wiring structure of FIG. 10A.
The structure of FIG. 10C is an electrode arrangement that has sixfold symmetry, and therefore has less chance of peeling with respect to a particular direction. The structure of FIG. 10C is also improved in the ease of embedding of the humidity sensitive film because the humidity sensitive film is pinched more lightly between the electrodes 3a and 3b than as in FIG. 10A and FIG. 10B. However, the structure of FIG. 10C is not higher in capacitance per unit area than the comb-teeth wiring structure as described later, despite the fact that the columnar electrodes are formed at the closest packing density. In addition, the manufacturing cost rises in the case where the columnar electrodes 3a and 3b are manufactured by a dedicated process (see Japanese Patent Application Laid-open No. 2014-167445, for example) that may not match with a normal semiconductor manufacturing process. In the case where signal processing circuits and the humidity sensor are built on the same semiconductor chip, forming the electrodes 3a and 3b of the humidity sensor in a step of manufacturing wiring of the signal processing circuits is desirable in order to keep the manufacturing cost from increasing.